Prokaryotic and eukaryotic Cell

Course : B.Sc. Biotechnology, BiochemistrySem IISub: Basic MicrobiologyUnit 1.3

Prokaryotic cell• A prokaryote is a single-celled organism that lacks a membrane-bound

nucleus (karyon), mitochondria, or any other membrane-bound organelles.

• The word prokaryote comes from the Greek pro- "before" and karyon- "nut or kernel".

• All the intracellular water-soluble components (proteins, DNA and metabolites) are located together in the same volume enclosed by the cell membrane, rather than in separate cellular compartments.

• Prokaryotes, however, do possess protein-based microcompartments, which are thought to act as primitive organelles (protein-bounded and lipid-bounded organelles).

• Some prokaryotes also have multicellular stages in their life cycles, such as myxobacteria, or create large colonies, like cyanobacteria.

Prokaryotic cell

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Eukaryotic cell• A eukaryote is any organism whose cells contain a nucleus and other

structures (organelles) enclosed within membranes.• Eukaryotes are formally the taxon Eukarya or Eukaryota. • The defining membrane-bound structure that sets eukaryotic cells

apart from prokaryotic cells is the nucleus, enclosed by the nuclear envelope, which contains the genetic material.

• The presence of a nucleus gives eukaryotes their name, which comes from the Greek eu, "well" and karyon, "nut" or "kernel".

• Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria or the Golgi apparatus.

• In addition, plants and algae contain chloroplasts. • Many unicellular organisms are eukaryotes, such as protozoa. • All multicellular organisms are eukaryotes, including animals, plants

and fungi.

Plant cell

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Animal Cell

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Comparison of eukaryotes and prokaryotes  Eukaryotic Cell Prokaryotic CellNucleus Present AbsentNumber of chromosomes More than one One--but not true

chromosome: Plasmids

Cell Type Usually multicellularUsually unicellular (some cyanobacteria may be multicellular)

True Membrane bound Nucleus Present Absent

Example Animals and Plants Bacteria and ArchaeaGenetic Recombination

Meiosis and fusion of gametes

Partial, undirectional transfers DNA

Lysosomes and peroxisomes Present Absent

Microtubules Present Absent or rare

…cont.Eukaryotic Cell Prokaryotic Cell

Endoplasmic reticulum Present Absent

Mitochondria Present AbsentCytoskeleton Present May be absent

DNA wrapping on proteins.

Eukaryotes wrap their DNA around proteins called histones.

Multiple proteins act together to fold and condense prokaryotic DNA. Folded DNA is then organized into a variety of conformations that are supercoiled and wound around tetramers of the HU protein.

Ribosomes larger smallerVesicles Present PresentGolgi apparatus Present Absent

Chloroplasts Present (in plants) Absent; chlorophyll scattered in the cytoplasm

Eukaryotic Cell Prokaryotic Cell

Flagella

Microscopic in size; membrane bound; usually arranged as nine doublets surrounding two singlets

Submicroscopic in size, composed of only one fiber

Permeability of Nuclear Membrane Selective not present

Plasma membrane with steroid Yes Usually no

Cell wall Only in plant cells and fungi (chemically simpler)

Usually chemically complexed

Vacuoles Present PresentCell size 10-100µm 1-10µm

…cont.

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Genetic material• Genes govern the properties of cells, and a cell's complement

of genes is called its genome. • DNA is arranged in cells to form chromosomes. • In prokaryotes, there is usually a single circular chromosome;

whereas in eukaryotes, several linear chromosomes exist.• Plasmids are circular extra chromosomal genetic elements

(DNA), nonessential for growth, found in prokaryotes.• The nucleus is a membrane-enclosed structure that contains

the chromosomes in eukaryotic cells. The nucleoid, in contrast, is the aggregated mass of DNA that constitutes the chromosome of cells of Bacteria and Archaea.

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• Universe formed 15 billion years ago (Big Bang)

• Galaxies formed from stars, dust and gas

• Earth formed 4.6 billion years ago

• Suns energy stripped away 1st atmosphere

• 2nd atmosphere formed from volcanic outgassing

• Primitive atmosphere: CO2, water vapor, lesser amts

of CO, N2, H2, HCl, and traces of NH3 and CH4 (3.5 bya)

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• O2 came in 1.5 bya

• Autotrophic Organisms: photosynthesis

• Another environmental change

• Result in evolution

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• 0.5 billion years ago

• Atmosphere O2 to 1% current

• Compare to present: 78% N2, 21% O2, 0.04% CO2, + trace gasses

• Relatively small, most single cell

• Start of multicellularity

• Increase in cell complexity

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Life began~ 3.5 bya

Organic molecules (C H O N P S) swimming in shallow seas

Stage 1: Abiotic synthesis of organic molecules such as proteins, amino acids and nucleotidesStage 2: joining of small molecules (monomers) into large moleculesStage 3: origin of self-replicating molecules that eventually made inheritance possibleStage 4: packaging these molecules into pre-cells, droplets of molecules with membranes that maintained an internal chemistry

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MAJOR EPISODES IN THE HISTORY OF LIFE

– Earth was formed about 4.6 billion years ago. – Prokaryotes• Evolved by 3.5 billion years ago• Began oxygen production about 2.7 billion years ago• Lived alone for almost 2 billion years• Continue in great abundance today

– Single-celled eukaryotes first evolved about 2.1 billion years ago.

– Multicellular eukaryotes first evolved at least 1.2 billion years ago.

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Major episode Millions of years ago

All major animal phyla establishedPlants and fungi colonize land

Origin of Earth

First multicellular organismsOldest eukaryotic fossils

Accumulation of O2 in atmosphereOldest prokaryotic fossils

5005301,2001,8002,4003,5004,600

Figure 15.UN03

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Resolving the Biogenesis Paradox

– All life today arises by the reproduction of preexisting life, or biogenesis.

– If this is true, how could the first organisms arise?– From the time of the ancient Greeks until well into

the 19th century, it was commonly believed that life regularly arises from nonliving matter, an idea called spontaneous generation.

– Today, most biologists think it is possible that life on early Earth produced simple cells by chemical and physical processes.

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A Four-Stage Hypothesis for the Origin of Life

– According to one hypothesis, the first organisms were products of chemical evolution in four stages.

– Stage 1: Abiotic Synthesis of Organic Monomers• The first stage in the origin of life has been the most

extensively studied by scientists in the laboratory.

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The Process of Science: Can BiologicalMonomers Form Spontaneously?

– Observation: Modern biological macromolecules are all composed of elements that were present in abundance on the early Earth.

– Question: Could biological molecules arise spontaneously under conditions like those on the early Earth?

– Hypothesis: A closed system designed in the laboratory to simulate early Earth conditions could produce biologically important organic molecules from inorganic ingredients. 

– Prediction: Organic molecules would form and accumulate.

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The Process of Science: Can BiologicalMonomers Form Spontaneously?

– Experiment: An apparatus was built to mimic the early Earth atmosphere and included• Hydrogen gas (H2), methane (CH4), ammonia (NH3), and

water vapor (H2O)• Sparks were discharged into the chamber to mimic the

prevalent lightning of the early Earth• A condenser to cool the atmosphere, causing water and

dissolved compounds to “rain” into the miniature “sea”

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The Process of Science: Can BiologicalMonomers Form Spontaneously?

– Results: After the apparatus had run for a week, an abundance of organic molecules essential for life had collected in the “sea,” including amino acids, the monomers of proteins.

– Since Miller and Urey’s experiments, laboratory analogues of the primeval Earth have produced• All 20 amino acids • Several sugars • Lipids• Purine and pyrimidine bases (found in DNA, RNA & ATP)

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Stage 2: Abiotic Synthesis of Polymers

– Researchers have brought about the polymerization of monomers to form polymers, such as proteins and nucleic acids, by dripping solutions of organic monomers onto• Hot sand• Clay• Rock

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Stage 3: Formation of Pre-Cells

– A key step in the origin of life was the isolation of a collection of abiotically created molecules within a membrane.

– Laboratory experiments demonstrate that pre-cells could have formed spontaneously from abiotically produced organic compounds.

– Such pre-cells produced in the laboratory display some lifelike properties. They:• Have a selectively permeable surface• Can grow by absorbing molecules from their surroundings• Swell or shrink when placed in solutions of different salt

concentrations

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Stage 4: Origin of Self-Replicating Molecules

– Life is defined partly by the process of inheritance, which is based on self-replicating molecules.

– One hypothesis is that the first genes were short strands of RNA that replicated themselves without the assistance of proteins, perhaps using RNAs that can act as enzymes, called ribozymes.

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From Chemical Evolution to Darwinian Evolution

– Over millions of years• Natural selection favored the most efficient pre-cells • The first prokaryotic cells evolved

– Prokaryotes lived and evolved all alone on Earth for 2 billion years before eukaryotes evolved.• Are found wherever there is life• Far outnumber eukaryotes• Can cause disease• Can be beneficial

– Prokaryotes live deep within the Earth and in habitats too cold, too hot, too salty, too acidic, or too alkaline for any eukaryote to survive.

References

Books:1. Biology of microorganisms By M. T. Madigan, J. M. Martinko, D. A. Stahl and D. P.

Clark

Websites:2. http://www.andavancollege.ac.in/depts/microbiology/mbnotes/fom.pdf

Images:3. http://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/

Average_prokaryote_cell-_en.svg/944px-Average_prokaryote_cell-_en.svg.png 4. http://img2.wikia.nocookie.net/__cb20120115004622/

horticultureandsoilscience/images/b/b8/649px-Plant_cell_structure_svg.svg.png 5. http://upload.wikimedia.org/wikipedia/commons/thumb/4/48/

Animal_cell_structure_en.svg/724px-Animal_cell_structure_en.svg.png 6. https://lh3.ggpht.com/

PYx4xlRmogHOYNgBhfL4_nApocJog5NlGb9ByHDHeBpzsCdVDSGOGamuxAhieJIlpE74RRg=s93